Porphyria cutanea tarda (PCT) is the most common porphyria. Subnormal activity of uroporphyrinogen decarboxylase (URODECARB) is a consistent finding in all patients with PCT. In some families, the enzyme deficiency is inherited as an autosomal dominant trait. Other cases of PCT have been associated with exposure to environmental toxins. The major objectives of this project are to determine the genetic mechanisms involved in the pathogenesis of PCT and to examine the regulatory role of the cytosolic heme synthetic enzymes in normal and disease states. Using a cloned human URODECARB gene as a probe, polymorphism at that locus will be sought. Family studies will be done, using this marker in a test of the hypothesis that an inherited abnormality at the URODECARB locus is responsible for PCT in many patients. URODECARB will be purified from patients with PCT to compare the kinetic behavior and amino acid sequence of the enzyme with that in normal subjects. We plan to explore the possibility that heredity may influence the metabolism of toxins which produce PCT, and thus susceptibility to toxin-induced PCT. Lymphocytes from resistant and susceptible mouse strains and from humans with and without PCT will be exposed to toxins in culture, and accumulation of porphyrins will be measured in an attempt to make a cell culture model of the disease. An adaptation of the Ames test for carcino- gens will be used to determine if liver homogenates from mice with toxin- induced porphria affect porphyrin metabolism in culture human cells. Similar experiments will be performed with human porphyric liver and purified URODECARB. Three cytosolic enzymes, porphobilinogen deaminase, uroporphyringen III cosynthase, and URODECARB, catalyze consecutive steps in porphyrin synthesis. We will purify each of these enzymes in order to determine if specific interactions between the three proteins might have a regulatory function in heme biosynthesis.